As a system for observing, for example, smoke exhausted from a chimney of a power station or the like, technologies disclosed in Patent Document 1 and Patent Document 2 are known. These technologies use a plurality of ITVs or color cameras to detect the presence or absence of smoke exhausted from the chimney using parallax and color difference between the cameras.
Patent Document 1: Japanese Patent Application Laid-open No. S63-88428
Patent Document 2: Japanese Patent Application Laid-open No. H10-232198